The invention relates to semiconductor structures, particularly to high electron mobility transistors (HEMTs).
There are several types of field effect transistors (FETs) that can be used at microwave and millimeter wave frequencies. One of these FETs includes a high electron mobility transistor (HEMT), which can be formed from Group III-V materials such as gallium arsenide (GaAs) and indium phosphide (InP).
Generally, a HEMT includes a donor/barrier layer and a channel layer. The donor/barrier layer is generally a wide-band gap material, and the channel layer is generally a lower-band gap material. A heterojunction is typically formed between the donor/barrier and the channel layers. Due to a conduction band discontinuity at the heterojunction, electrons are injected from the donor/barrier layer into the channel layer. Electrons injected into the channel layer are confined to move in a plane parallel to the heterojunction due to the relatively larger bandgap of the donor/barrier layer. Consequently, there is a spatial separation between dopant atoms in the donor/barrier layer and electrons in the channel layer, which results in low impurity scattering and good electron mobility. It is generally desirable for HEMTs to have high power performances, high breakdown voltages, and high current densities.
As a channel layer in a transistor structure for microwave power and millimeterwave operations, InP has high saturated velocity, moderate mobility, and high breakdown field. However, InP has a low Schottky barrier height. Furthermore, using AlInP as a Schottky layer in a FET or as a donor/Schottky layer in a HEMT structure can be limited because elastic strain can limit the aluminum concentration of AlInP on InP substrates to approximately 15%. Consequently, the Schottky barrier and HEMT conduction band discontinuity may only be modestly improved. Elastic strain can also limit a HEMT structure to a single-sided AlInP/InP heterojunction because the combined tensile strain of having two AlInP layers in a double-sided AlInP/InP/AlInP HEMT could exceed elastic strain limits and cause device-degrading crystalline dislocations. Moreover, an additional limitation is growth on InP substrates which can be more expensive, smaller, and more fragile than GaAs substrates.
In accordance with the invention, a double pulse doped semiconductor structure, e.g., a HEMT, having two AlxIn1xe2x88x92xP donor/barrier layers and an InP channel layer is provided. Generally, the structure is formed by using metamorphic growth and strain compensation. A metamorphic graded layer and a relaxed buffer layer are formed on a first substrate, e.g., GaAs and InP, to provide a xe2x80x9cnew substratexe2x80x9d having a lattice constant different than the lattice constant of the first substrate. The lattice constant of the relaxed buffer layer is intermediate the lattice constants of the donor/barrier layers and the channel layer. When the donor/barrier layers and the channel layer are formed on the relaxed buffer layer, these layers develop strain due to the differences in lattice constants of these layers and the relaxed buffer layer. However, these strains compensate for each other to near equilibrium, thereby allowing the donor/barrier and channel layers to be formed with minimized crystalline dislocations. From a cross-sectional transmission electron microscope (TEM) micrograph, the layers of the structure exhibit good planarity and threading dislocations are not readily apparent. From plan view TEM micrographs, the dislocation density is estimated to be less than 1xc3x97106 cmxe2x88x922.
The structure of the invention includes an InP channel layer with good, practical thicknesses and two AlxIn1xe2x88x92xP donor/barrier layers with relatively high aluminum concentrations that provide two AlInP/InP heterojunctions. By using strain-compensated AlInP/InP layers on top of a relaxed buffer layer provided by metamorphic growth, AlInP donor/barrier layers can be grown pseudomorphically with up to 40% aluminum concentration. This concentration is approximately twice that of some growth on InP substrates. Alloying aluminum into InP, up to about 40%, increases the bandgap of InP, e.g., from about 1.35 eV for InP to about 2.03 eV for Al0.30In0.70P. High bandgaps provide high breakdown characteristics, which allow high breakdown devices to be formed. Large bandgap donor/barrier layers also provide good charge transfer into the channel layer and good confinement of current in the channel layer. Increasing the aluminum concentration also increases the Schottky barrier height. Furthermore, the structure of the invention also includes heterojunctions, e.g., Al0.30In0.70P/InP, that have relatively high conduction band discontinuities between the channel layer and the donor/barrier layers. High conduction band discontinuity provides high current density and good charge confinement of current. Metamorphic grading also permits growth of this structure on GaAs substrates, which can be larger, less expensive, and more robust than InP substrates.
In one aspect, the invention features a semiconductor structure, e.g., a high electron mobility transistor structure, including a substrate, a graded layer, a first donor/barrier layer, and a channel layer. The, substrate (e.g., InP and GaAs) has a substrate lattice constant. The graded layer (e.g., (AlGa)0.25In0.75P) is disposed over the substrate and has a graded lattice constant, wherein the graded layer has a first lattice constant near a bottom of the graded layer substantially equal to the substrate lattice constant and a second lattice constant near a top of the graded layer different than the first lattice constant. The first donor/barrier layer (e.g., Al0.30In0.70P) is disposed over the graded layer and has a third lattice constant, and the channel layer (e.g., InP) is disposed over the first donor/barrier layer and has a fourth lattice constant. The second lattice constant is intermediate the third and fourth lattice constants. The structure can further include a relaxed buffer layer (e.g., (AlGa)0.20In0.80P) over the graded layer and having a fifth lattice constant intermediate the third and fourth lattice constants, e.g., larger than the third lattice constant.
Embodiments of the invention may include one or more of the following features. The second lattice constant is smaller than the first lattice constant and/or larger than the third lattice constant. The first donor/barrier layer is tensilely strained and the channel layer is compressively strained. The strains in the first donor/barrier layer and the channel layer are substantially at equilibrium. The lattice constant of a portion of the graded layer varies with distance from the substrate. The graded layer includes a Group III-V material having a Group III material with a first concentration of indium at the bottom of the graded layer higher than a second indium concentration at the top of the graded layer. The graded layer has a linearly varying indium concentration or a stepwise indium concentration. The difference between the second and third indium concentrations is between about 3 percentage points and about 8 percentage points, e.g., about 5 percentage points.
The first donor/barrier layer can include a Group III-V material having a Group III material with a fourth indium concentration lower than the second indium concentration. The first donor/barrier layer includes a Group III-V material having a Group III material with an aluminum concentration between about 23% and about 40%. The channel layer has a thickness of about 80 xc3x85 to about 130 xc3x85.
The structure can further include over the channel layer a second donor/barrier layer wherein the second donor/barrier layer includes a Group III-V material having a Group III material with an aluminum concentration between about 23% and about 40%. The structure can further include a selectively-etchable contact layer (e.g., InwGa1xe2x88x92wAs) over the second donor/barrier layer.
In another aspect, the invention features a high electron mobility transistor structure including a GaAs substrate having a substrate lattice constant, a graded layer over the substrate and having a graded lattice constant, a first relaxed buffer layer over the graded layer and having a third lattice constant, a second relaxed buffer layer over the first relaxed buffer layer and having a fourth lattice constant substantially equal to the third lattice constant, a first donor/barrier layer over the second relaxed buffer layer (e.g., (AlGa)nIn1xe2x88x92nP) and having a fifth lattice constant, and a channel layer over the first donor/barrier layer and having a sixth lattice constant. The graded layer has a first lattice constant near a bottom of the graded layer substantially equal to the substrate lattice constant and a second lattice constant near a top of the graded layer different than the first lattice constant. The first relaxed buffer layer includes a Group III-V material comprising arsenic, and the second relaxed buffer layer includes a Group III-V material comprising phosphorus. The fourth lattice constant is intermediate the fifth and sixth lattice constants. The structure can further include over the channel layer a second donor/barrier layer wherein the second donor/barrier layer includes a Group III-V material having a Group III material having an aluminum concentration between about 23% and about 40%. The structure can further include a selectively-etchable contact layer (e.g., InwGa1xe2x88x92wAs) over the second donor/barrier layer.
Embodiments of the invention may include one or more of the following features. The fourth lattice constant is larger than the fifth lattice constant. The graded layer comprises a Group III-V material includes arsenic, e.g. (AlGa)xIn1xe2x88x92xAs, where x is between about 0.6 and about 1. The graded layer includes a Group III-V material having a Group III material with a first concentration of indium at the bottom of the graded layer lower than a second indium concentration at the top of the graded layer, and the first relaxed buffer layer includes a Group III-V material having a Group III material having a third indium concentration lower than the second indium concentration. The difference between the second and third indium concentrations is between about 3 percentage points and about 8 percentage points. The first donor/barrier layer includes a Group III-V material having a Group III material having an aluminum concentration between about 23% and about 40%.
With these configurations, the structures can be formed with either one or two arsenic/phosphorus (As/P) interfaces that are not in the high field regions of structures. Forming these interfaces remote from the high field region can lead to better performance of a structure because formation of abrupt, defect-free As/P interfaces can be difficult and defects at these interfaces can provide a leakage path between a gate and a drain that can lead to premature breakdown of the structure.
As used herein, xe2x80x9c(AlGa)xxe2x80x9d means Alx, Gax, or AlyGax-y. For example, (AlGa)0.25In0.75P means Al0.25In0.75P, Ga0.25In0.75P, or AlyGa0.25xe2x88x92yIn0.75P, where 0xe2x89xa6yxe2x89xa60.25.
Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.